The primary purpose of the current study was to examine the writing performance and the use of socio-cognitive strategies among monolingual Persian and bilingual Turkish-Persian EFL learners across two proficiency levels. A mixed-method approach was employed to analyze the data gathered from the quantitative analysis of socio-affective writing strategy questionnaires as well as qualitative analysis of sample written texts and semi-structured interviews. Results of a series of two-way ANOVAs showed that bilinguals used more positive affective strategies than monolinguals. While the difference between high-proficient and low-proficient participants in strategy use was not significant, some differences were found between bilinguals and monolinguals across different proficiency levels concerning their overall writing quality. The findings also highlighted the underestimation of writing socio-affective strategy instruction, which can be considered as one of the reasons contributing to the writing problems of EFL learners.